To help prevent failure in the foundation of water carrying utilities, it is necessary to know the conditions of concrete substructures, both existing and under development. The structural integrity of the concrete is also determined as an indicium of acceptance and as a safety assurance measurement for soundness of structural support.
In the prior art of which we are aware, non-destructive testing of concrete is made by impacting the concrete to induce vibrations and then measuring the magnitude of vibrations at positions on the concrete spaced apart from the point of impact. For example, Brands U.S. Pat. No. 3,888,108, issued on June 10, 1975, discloses a pavement testing apparatus including a mallet that is adapted to impact the pavement and an accelerometer spaced apart from the mallet to monitor vibrations in the pavement at a single position spaced apart from the mallet. The mallet and accelerometer are both carried within a small, portable container that rests on the pavement surface.
Another type of pavement testing apparatus is disclosed in Milburger et al. U.S. Pat. No. 3,762,496, issued on Oct. 2, 1973, wherein a tri-wheel cart contains a metal-pegged wheel that successively taps the pavement as the cart is wheeled, and induced vibrational energy in the pavement is monitored by transducers at two of the wheels. The transducer wheels and pavement tapping wheel are positioned on a common axis.
While the prior art apparatus for testing pavement of which we are aware have been generally satisfactory for limited applications, these apparatus are not suitable for monitoring very fine voids or discontinuities behind concrete linings and at concrete separations because the single transducer or linear transducer array does not provide adequate sensitivity. Fine voids or discontinuities in concrete substructures for heavy-duty applications, such as water carrying utilities, tend to grow under pressure and fail after a period of time. Also, testing a substantial concrete area using the prior art apparatus of which we are aware requires a substantial testing period since only a small area of the concrete is monitored at any time by the single transducer or linear transducer array.
Accordingly, one object of the invention is to provide an improved method of and apparatus for testing concrete that is responsive to very fine structural defects.
Another object is to provide an improved method of and apparatus for testing concrete wherein the concrete is monitored with a relatively wide field of view so that a large area of concrete can be scanned in a relatively short period of time.
Another object is to provide a new and improved method and apparatus, wherein concrete is non-destructively tested by impacting the concrete to induce vibrations and monitoring vibrational energy in the concrete using an array of three transducers equispaced from the point of impact.
Another object is to provide a method of and apparatus for non-destructively testing concrete, wherein a portion of the concrete is impacted to induce vibrations and vibrational energy in the concrete is monitored by an array of transducers mechanically coupled to the concrete and positioned in a triangular array with the point of impact of the concrete being equispaced from the transducers.
Another object is to provide a method of and apparatus for non-destructively testing concrete, wherein a portion of the concrete is impacted to induce vibrations, and vibrational energy in the concrete is monitored by first, second and third transducers oriented in a triangular array; and vibration responsive signals generated by the transducers are mutually compared, with any imbalance among the signals being characteristic of a void or discontinuity in the concrete.
Another object is to provide a new and improved apparatus for testing concrete that is portable, easy to position over a test region and adapted for use in confined areas of the types typically encountered at water carrying utilities.